This invention relates to a cathode-ray tube electron gun, and more particularly, to a method and structure for aging the cathode of a cathode-ray tube electron gun to obtain the desired level of cathode emission and tube performance.
It is well known in the art that thermionic cathodes require an "activation" or "aging" step to develop reproducible cathode emmissivity. A typical activation procedure requires that an overvoltage be applied to the filament heater in order to raise the cathode temperature substantially above the normal operating temperature. At the same time an accelerating potential, usually anode potential, is applied to the tube. The activation step is usually continued for a fixed period of time, the activation time being determined by a trial and error evaluation of the tubes. Such an activation procedure is described in U.S. Pat. No. 1,881,645 issued to Jones et al., on Oct. 11, 1932, in U.S. Pat. No. 1,983,668 issued to Jones et al., on Dec. 11, 1934, and in U.S. Pat. No. 2,561,768 issued to Adler, on July 24, 1951.
U.S. Pat. No. 3,357,766 issued to Conger on Dec. 12, 1967 describes an improved method of cathode activation in which the filament heater voltage is reduced from the initial overvoltage while the voltage of one of the prefocus grids is increased. A predetermined sequence of voltages and time intervals is utilized during the activation process.
In each of the above-described activation processes, there is no method for determining whether the desired level of cathode emission has been obtained until the tube is tested. In some tubes the cathodes are "underaged" and the activation process must be repeated. In other tubes the cathodes are "overaged" and some of the low work function cathode material is deposited on the next adjacent electrode, commonly known as the control or G.sub.1 electrode, of the electron gun. This deposit of low work function material causes emission from the control grid which appears as a low level area of illumination on the screen of the cathode-ray tube. In tubes such as photorecording cathode-ray tubes used, for example, in CAT scanners, where the screen is photographed, the grid emission induced screen illumination degrades the performance of the tube and must be eliminated.